This invention relates generally to vehicle theft alarms and more particularly, is directed to provision of vibration-sensitive trip switch means combining specific independent shock and motion responsive spring means whereby to effect predetermined insensitivity to shock and/or motion conventionally not associated with acts of theft or vandalism.
Vehicle alarm systems have become widely available to provide both audible and silent alarm in the event attempts are made to steal or otherwise tamper with a vehicle on which the alarm system operatively is installed.
One considerable disadvantage of available vehicle alarm systems is their undue sensitivity so as to sound due to other than theft, deliberate tampering, unauthorized motion etc. Often the alarm is tripped by reaction to minor shocks, strong wind gusts, heavy rain, hail, undue temperature sensitivity etc. Available trip switch means simply are too sensitive or not sufficiently sensitive.
Attempts to reduce the over-sensitivity of the alarm mechanism avoiding such accidental setting off of the alarm often have resulted in the alarm becoming insensitive to those intrusions primarily evidentiary of or preliminary to acts of theft or vandalism. For example, the skill of the vehicle thief has reached a level enabling access to be gained to the interior of a vehicle with very slight jarring impact, say upon a rear or side window as by using a thin flat bar to manipulate the locking device. Impact involved in breaking through a side window is sharp, most often brief and requires instantaneous response in energizing the alarm system. Alarms reactive to such occurrence often are also over-sensitive. Yet it is not feasible for the switch means to be also overly sensitive--so sensitive that a mere tap will set off the alarm.
Compromise is essential, yet available responsive means either are overly sensitive or not sufficiently sensitive. There are two distinct exigencies which demand specific responsive devices each functioning independently to detect a materially distinct event. For example, independent shock detectors and independent motion detectors are required. These detector units are materially different in their sensitivity and often require separate sensing means and trip means responsive respectively to the different exigencies. Each unit is provided with its own housing, mounting means and lead connections to be made to the alarm system.
Conventionally, spring biased trip switch means are provided with a sensitivity within a narrow range so as to detect, react and trip the alarm should a specific event occur. Often a rigid band spring is arranged to be flexed by the jarring, vibration or motion of the vehicle--a stationary contact disposed below same. The reaction of the spring effects an electrical connection to alarm means upon occurrence of the event.
Thin metal connector strips have a tendency to warp under the somewhat high temperatures encountered during operation of the vehicle or even under some other adverse environmental conditions. Extreme heat or cold effects the reactivity, i.e., bendability, flexibility, vibrational response, etc. Changes in thermal and/or electrical conductivities under different operating conditions often effect the sensitivity of the alarm switch means available presently.
Not only are problems encountered with the over and under sensitivity of available switch means. It is essential that the electrical connection established upon occurrence of the specific event be positive and brief. The electrical contacts must not stick together once the initial contact has been established. Normally, the spring bias of the flexible member is sufficient to open the tripped switch. Under extreme temperature conditions, the contacts have a tendency to retain their engagement. The springs may lose their resilience or the contact material may be softened so as to overcome the bias of the spring. The spring may become warped or twisted preventing its return to pretrip condition or even to prevent the contact from being made initially on the occurrence of the event.
Most alarm systems have time controls, delays, etc. so that once energized, the duration of the alarm, whether it be a siren, a personal or home beeper or a signal delivered to a monitoring agency such as a low enforcement unit, is limited. Further, once the system is deenergized after an event, as by a key operated means, continued engagement of the switch contact means would prevent deenergization with considerable damage to the system ensuing. Thus, instant-on instant-off characteristics are desirable.